Method and apparatus for forming a heat exchanger tube with closely spaced integral fins

ABSTRACT

Fins are formed on a copper heat exchanger tube by a plurality of rolling dies which roll grooves into the surface of the tube. The tube walls are supported by an internal mandrel. The pitch between the roll dies generally increases but at a decreasing rate from the lead roll die to the trailing roll die to accommodate axial elongation of the tube resulting from the radially pressures exerted on the tube by the roll dies and mandrel.

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[56] Retell-emcee Cited UNITED STATES PATENTS [54] ME'll'llllUlD AND APPAMATUfi i on lFQRMlNG A ll-lllllfit'll EXCHANGIER 'lllllElE Wl'llllll CLQSlElLSY SPAUEHB llN'lllEGlh/hk lFlNfi [72] Inventors:

Schuster.............................

William A. llfilug, La Crosse; Andrew A. llllabiger, La Crescent; Leonard .ll. Malrl, La. Primary EXaml'1er'-LWellA-Laf$n Crosse, all of Wis. AttorneyArthur 0. Andersen and Carl M. Lewis ABSCT [73] Assignee: The 'llrnne Company, La Crosse, Wis.

[22] Filed. Fins are formed on a copper heat exchanger tube by a plurality of rolling dies which roll grooves into the surface of the Mar-.41, m

tube. The tube walls are supported by an internal mandrel. [21] Appl. No.: 116,274 The pitch between the roll dies generally increases but at a decreasing rate from the lead roll die to the trailing roll die to accommodate axial elongation of the tube resulting from the [52] US. Cl. 72/98, 29/1573 All radially pressures exerted on the tube by the roll dies and mandrel.

[51] Int. ill/1B2 .72/96, 98,100,103,104,l07, 72/108, 78; 29/1573 AH; 113/118 A m e ew wiet ewew [58] lFfieltll all Search...............

LAB A7 11 6 g '4 INVENTOR. WILL A. KLUG BY ANDR A. HABIGER LEONARD J. BAHL ATTORNEY METHOD AND APPARATUS FOR FORMING A HEAT EXCHANGIEIR TUBE WTTH CLOSIELY SPACIEID TNTEGRAL IFINS This invention relates to apparatus and method for manufacturing finned tubing such as used for shell-and-tube type heat exchangers, as for instance, condensers and evaporators for refrigeration machines.

One method for forming integral fins on copper tubing has been to circumferentially groove the outer portion of the tube and then axially compress the material between adjacent convolutions of the groove to develop therefrom radially projecting fins. These operations are usually performed by rollers distributed about the tube in rolling intersecting relationship therewith, each roller having a series of axially spaced roll dies which are progressively increased in width with a correspond ing decrease in width of the spaces or fins therebetween from the leading die to the trailing die. The axes of the rollers are normally at a slight angle to the axis of the tube and thus a helical fin is extruded up from the surface of the tube.

For heat exchangers of the shell-and-tube type, it is generally desirable to leave the tube unfinned in the areas adjacent the tube sheets and tube supports. Furthermore, it is desirable that the fins extend radially outward no further than the outer diameter of the original tube so that the finned tube may be freely inserted through the apertures in the tube sheets and tube supports. For this reason, it is now customary in equipment of this type to provide heat exchanger tubes constructed by a similar method wherein the fins are formed by rolling grooves into the surface of the tube with a substantial simultaneous reduction in tube diameter. While the helix angle changes in the transition to a reduced diameter the roll dies are disposed at a common angle and pitch. The fin diameter is normally limited to less than the original tube diameter.

Tubes made by this general method having a diameter of about /8 to about 1 inch are commercially available now with fin densities up to about 25 fins per inch. However, when we attempted to make a tube having a fin density of about 35 fins per inch using only conventional methods, an inferior fin was produced. We found that the dies tended to mistrack thus producing fins which were under cut, had laps, were inclined, were notched in the grooves, lacked uniform spacing, had metallic slivers, and had nonuniform radii at their base. After careful investigation it is believed that these defects resulted from tube elongation, especially between adjacent convolutions of the fins. The elongation is produced by the excess material made available in the forming of grooves into the tube surface and simultaneous reduction in tube diameter. Elongation of tubes in the area of the finning dies has long been recognized as a problem. One method to eleviate this problem has been to provide the tube with a twist, simultaneously with the finning operation which it is believed tends to effect a shortening of the tube to compensate for the elongating forces. Another method employed to obviate this problem has been to arrange the groove forming roll dies into two or more axially spaced groups. The tube is said to be stressed relieved in the area between the sets of axially spaced dies.

The instant invention pertains to another method and apparatus for obviating the problems of tube elongation accompanying the formation of grooves within the tube surface. By our novel method tubes having a high quality profile and spacing have been produced in fin densities in excess of 35 fins per inch. This is accomplished by providing a progressively increasing pitch between roll dies on each roll assembly. In other words, the groove in the tube surface is formed by a procession of roll dies which by their disposition provide for progressive increases in the pitch of the groove which are of such magnitude as to be commensurate with the growth or elongation of the tube in the vicinity ofthe roll dies.

It is thus an object of this invention to provide an improved means for constructing integral fin heat exchanger tubes.

It is a further object of this invention to provide a method and apparatus for producing heat exchanger tubes with a high quality integral fin surface.

It is still another object of this invention to provide a method and apparatus for producing heat exchanger tubes of the integral fin type having a high fin density in the order of 30 or more fins per inch.

And another object of this invention is to provide the method and apparatus for producing integral fin heat exchanger tubes having extremely uniform fin spacing.

And it is still another object of this invention to provide a method and apparatus means to prevent the mistracking of fin forming dies during the formation of integral fin heat exchange tubes.

Other objects and advantages will become apparent as this specification proceeds to describe the invention with reference to the drawings in which:

FIG. 1 is an enlarged and exaggerated section taken substantially at line 1-1 of FIG. 2 showing the tube finning apparatus in the process of forming integral fins upon a tubular blank;

FIG. 2 is an enlarged and exaggerated end view of the tubular blank and tube finning apparatus shown in FIG. 1; and

FIG. 3 is an exaggerated and further enlarged diagram showing the relative position of the groove forming roll dies of the tube finning apparatus of FIGS. 1 and 2.

The change in pitch between adjacent roll dies had been exaggerated by a factor of about 10 in FIGS. 1, 2 and 3 so as to more clearly illustrate the invention.

Now referring specifically to FIGS. l and 2, it will be seen that the tube finning apparatus 10 is comprised of an internal mandrel 11 disposed within an elongated copper tubular blank 12 of circular cross section and an external groove forming head 13.

Head 13 has a cylindrical body 14 which has 5 common central axis 9 with mandrel 11 and tubular blank 12. A forward annular plate 15 is disposed at the forward end of cylindrical body 14. Plate 15 has a plurality of arculate slots 16 circumferentially arranged about the common axis 9. Axially extending bolts 17 disposed within slots 16 and threaded into cylindrical body 14 adjustably secure forward annular plate 15 to the forward end of cylindrical body 141. It will be appreciated that by temporarily loosening bolts 17, forward annular plate 15 may be adjustably rotated relative to cylindrical body 14 about axis 9. In like manner a rear annular plate 18 is adjustably secured to the rear end of cylindrical body 14 by a plurality of axially extending bolts 19 extending through similar arcuate slots 20. Each of forward and rear annular plates 15 and 18 has four equally spaced radially inwardly extending support bars 21 and 22 adjustalbly secured within forward and rear radially extending bores 23 and 24 respectively by set screws 25. Bars 21 and 22 and bores 23 and 24 are circular in cross section whereby bars 21 and 22 may be adjustably rotated for reasons herein after described. At the radially inward end of each of bars 21 and 22, and preferable integral therewith, is a bearing support ring 26 and 2? respectively. A bearing 28 is supported in each ring 26 and a bearing 35 is supported in each ring 27. Each of bearings 28 and 35 has both radial and axial thrust bearing surfaces.

Roll assemblies A, B, C, and D are joumaled between pairs of bearings 2b and 35 at front and rear ends of head 13. Roll assembly A may be substantially identical to that of roll assembly C and roll assembly B may be substantially identical to roll assembly D. It therefore will suffice to describe in detail only roll assemblies A and B.

Roll assembly A shown in cross section in the upper portion of FIG. 1 includes an arbor 29 journaled at the front and rear ends thereof in a pair of bearings 23 and 35. A plurality of roll dies A A A A,,, A A A and A,, respectively are disposed from front to rear of arbor 29.. A plurality of annular spacer shims 30 may be disposed between the roll dies to provide the desired spacing. The roll dies and shims may be fixedly clamped to arbor 29 by arbor end bolts 31.

Each die is a hardened annular disk having an appropriate rounded profile at the periphery thereof for rolling a groove into tubular blank 12. It should be noted particularly that the dies A,, A A A A A A and A generally progressively increase in pitch from front to rear as well as in thickness and external diameter. The reasons for this progressive increase in pitch between the dies on arbor 29 from front to rear will be more clearly set forth hereinafter in connection with the operation of the tube finning apparatus.

Roll assembly B shown in cross section in the lower portion of FIG. 1 includes an arbor 32 having journals at the front and rear ends thereof in another pair of bearings 28 and 35. A plurality of roll dies B,, B B B B B B and B, respectively are disposed from front to rear on arbor 32. A plurality of annular spacer shims 33 may be disposed between the roll dies to provide the desired spacing. The roll dies and shims may be fixedly clamped to arbor 32 by arbor end bolts 34.

Each die on roll assembly B also is a hardened annular disk having an appropriate rounded profile at the periphery thereof for rolling a groove into tubular blank 12. It should be noted particularly that the dies B,, B B B B B B and B progressively increase in pitch from front to rear as well as in thickness and external diameter. The reasons for the progressive increase in pitch between the dies on arbor 32 from front to rear will be more clearly set forth hereinafter in connection with the operation of the tube finning apparatus.

It will be noted in FIG. 2 that roll assemblies A, B, C, and D are angled slightly so as to follow a helical path relative to tube 12. This is accomplished by rotatably advancing rear support bars 22 somewhat ahead of front support bars 21 in the direction of the rotation of head 13 relative to the direction of tubular blank 12.-This relative rotational adjustment of support bars 22 relative to support bars 21 may be accomplished by loosening the bolts 17 or 19 and loosening the set screws 25. The forward annular plate 15 may be rotated in slots 16 and/or the rearward annular plate 18 may be rotated in slots 20 to the desired degree after which said bolts and set screws may be retightened as aforedescribed. Support bars 21 and 22 will be slightly rotated in bores 23 and 24 during this adjustment. In this connection it should again be noted that the sectional view of FIG. 1 is not precisely taken as where indicated in FIG. 2 for purposes of simplicity. The section of FIG. 1 views the roll assemblies A and B along a plane passing through the axes thereof, i.e., the axes of arbors 29 and 32.

Furthermore, it should be noted that the dies of roll assembly B are axially disposed somewhat behind the respective dies of roll A. Thus die B, is disposed somewhat behind die A,, die B somewhat behind die A, and so on. In like manner, the dies or roll assembly D are disposed axially behind the respective dies of roll C. An understanding of the relative disposition of all the roll dies may be obtained by reference to the diagram of FIG. 3.

In this diagram each horizontal line represents a roll assembly. The position of the roll dies on each roll assembly is indicated by the intersection of the horizontal line and a oblique line. Thus it will be seen that the bottom horizontal line represents roll assembly A having dies A,, A A A A A A and A,,, as indicated by the intersections of the oblique lines with the bottom horizontal line. In similar manner the second, third and fourth horizontal lines from the bottom of 1 the diagram represent the die positions on roll assemblies B, C

and D respectively. The pattern has been repeated four times to represent four revolutions of head 13 relative to tubular blank 12. The solid oblique lines represent the sequence of dies passing in one groove while the dashed oblique lines represent the sequence of dies passing in the other groove. It will be understood that in this particular instance two grooves are formed whereby a double lead fin thread is formed on the surface of tubular blank 12. Thus starting at the lower righthand corner of the diagram, it will be seen that one groove is formed by the procession of dies A,, B,, C D A B C.,, D A 8,, C D A B C and D,,. In like manner the other groove is formed by the procession of dies C,, D,, A B C D A B C D A B C-,, D,, A and B The general curvature of the oblique lines on the diagram as constructed is entirely due to the change in pitch between adjacent dies both along the roll assembly arbors and along the groove path as the diagram does not take into consideration the change in helix angle due to the progressively reduced diameter of the groove.

To operate the tube finning apparatus 10, the tube is fed axially relative to the groove forming head 13 as indicated by the vectors in FIG. 1. The mandrel 11 may previously have been inserted in and carried along with the tubular blank 12, or it may be supported in axially fixed relationship with the groove forming head 13. In any event, the groove forming head is rotatably driven relative to the tubular blank 12 as indicated by the vectors in FIG. 2. Due to the inclination of the arbors of roll assemblies A, B, C and D, the groove forming head 13 drives itself axially relative to tubular blank 12 as indicated by the vectors in FIG. 1. It will be seen from FIG. I that the spaces between the convolutions of the grooves increase markedly in the forward half of the groove forming head 13 where the greatest amount of tube elongation is taking place. Thus it will be seen that as the tube elongates the groove spacing progressively increases. However, the die spacing or pitch also increases so that the dies are maintained in proper alignment with their respective grooves. For this reason the contacting point or working portion of the die remains centered at the bottom of the associated groove which in its development takes the form of a helicold of increasing pitch and decreasing diameter.

As herein before stated the change in pitch between the adjacent roll dies on each roll assembly has been greatly exaggerated for purposes of illustration and explanation in FIGS. 1, 2 and 3. In actual experiments conducted by us on approximately a three-fourth inch diameter copper tubular blank, a

double lead thread having a thread density of 35% fins per,

inch and a height of approximately thirty-thousandths of an inch was formed. The fins were formed into the tube, that is they were not extruded radially outward beyond the original outer diameter of the tube. About 15 percent axial elongation was experienced in the vicinity of the dies. in this successful experiment the dies were arranged in a similar order to that shown in FIGS. 1, 2 and 3 of the drawings. The distances of the dies from a plane 36 normal to the axis 9 of the tubular blank passing through the center of lead dies A, and C, was as follows:

A, and C, 0.0000 inches B, and D, 0.0120 inches A, and C, 0.0240 inches B and D, 0.0365 inches A and C, 0.0490 inches B; and D,, 0.0615 inches A, and C. 0.0745 inches B, and D, 0.0875 inches A, and C, 0.1010 inches B, and D, 0.1150 inches A, and C 0.1290 inches B, and D, 0.1430 inches A, and C, 0.1575 inches B, and D 0.1715 inches A and C, 0.1860 inches B, and D, 0.2000 inches The pitch between successive adjacent dies on roll assemblies A and C from front to rear was thus as follows:

.0240 inches .0250 inches .0255 inches .0265 inches .0280 inches .0285 inches .0285 inches M And the pitch between successiveadjacent dies on roll assemblies B and D from front to rear was thus as follows:

T623? in c11 es .0250 inches .0260 inches .0275 inches .0280 inches .0285 inches .0285 inches and B as shown by the clashed oblique lines of FIG. 3. The

pitch between succeeding di es along either of the two grooves from front to rear was as follows:

.0120 inches .0120 inches .0125 inches .0125 inches .0125 inches .0130 inches .0130 inches .0135 inches .0140 inches .0140 inches .0140 inches .0145 inches .0140 inches .0145 inches .0140 inches For purposes of illustration of FIGS. 11, 2 and 3 the actual changes in pitchwere multiplied by a factor of and then greatly enlarged to illustrate how the instant invention permits elongation of the tubular blank without damage to the roll dies or axial displacement of the fin. It will, of course, be appreciated that the amount of elongation will be depended upon a number of factors including the tubular blank dimensions of diameter and wall thickness, the tubular blank material, the final fin spacing, the final fin height, and the final tube diameter. With a reasonable amount of experience it is possible to predict to some degree of accuracy the amount of expected tube elongation for a desired finned tube configuration. Once the tube elongation is known, it is then possible to space the roll dies in such a manner as to provide for the change in pitch therebetween as herein disclosed.

When the dies are so arranged with increasing pitch therebetween to accommodate for the natural elongation of the tubular blank, fin densities of 35 fins per inch and more may be achieved without excessive die breakage of fin deformation.

Having now described the preferred method and apparatus for practicing our invention, we contemplate that many changes may be made without departing from the scope or spirit of our invention and we accordingly desire to be limited only by the claims.

We claim:

1. Apparatus for forming a helical finned tube comprising; means for supporting an elongated tubular blank; means for initially forming a shallow helical groove in the surface of the tube blank, and die means including work engaging portions for simultaneously changing the depth and pitch of said groove to accommodate changes of length of the tubular blank in the vicinity of engagement with said die means.

2. The apparatus as defined by claim I wherein said change of pitch ofsaid helicoidal path is achieved at least once in each of a plurality of successive convolutions.

3. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the pitch between the dies on separate roll assemblies and proceeding in a single groove in the outer surface of said tubular blank generally increases from the leading end toward the training end of the procession of dies in said single groove to accommodate elongation of said tubular blank in the vicinity of engagement with said roll assemblies.

4. ln apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality ofaxially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the pitch between the dies on each roll assembly generally increases from the leading end toward the trailing end of the roll assembly to accommodate elongation of said tubular blank in the vicinity of engagement with said roll assemblies.

5. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axia ll y s paced r oll dies adapted to rollingly engage the exterior surface of said Ebular blank, the improvement wherein the leading die of one roll assembly is displaced forwardly of the leading die of another roll assembly and the average pitch between the dies of said one roll assembly is less than the average pitch between the dies of said other roll assembly to thereby accommodate elongation of said. tubular blank in the vicinity of engagement with said roll assemblies.

6. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel. means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the average pitch between the dies in the forward half of each of said roll assemblies is less than the average pitch between the dies in the rear half of each of said roll assemblies to accommodate elongation of the tubular blank in the vicinity of engagement with said roll assemblies.

7. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the pitch between the dies on separate roll assemblies and proceeding in a single groove in the outer surface of said tubular blank differs to accommodate changes in length of the tubular blank in the vicinity of engagement with said roll assemblies.

0. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the pitch between the dies of one roll assembly differs from the pitch between the dies of another adjacent roll assembly to accomrnodate changes in length of the tubular blank in the vicinity of engagement with said roll assemblies.

9. A method of forming an integrally finned heat exchanger tube comprising the steps of: providing an elongated tubular blank of substantially circular cross section; supporting the internal surface of said tubular blank; and passing along a helical path relative to said tubular blank a series of fin extruding dies in engagement with the exterior surface of said tubular blank wherein the center of the areas of engagement lie substantially along a helicoidal path of decreasing diameter and increasing pitch to accommodate for elongation of said tubular blank in the vicinity of engagement with said dies wherein said axis of said helicoidal path is substantially coaxial with the axes of said helical path and said tubular blank.

10. A method of forming an integrally finned heat exchanger tube comprising the steps of: providing an elongated tubular blank of substantially circular cross section; supporting the internal surface of said tubular blank; and passing along a helical path relative to said tubular blank a series of roll dies in rolling engagement with the exterior surface of said tubular blank wherein the center of the areas of engagement lie substantially along a helicoidal path of decreasing diameter and increasing pitch to accommodate for elongation of said tubular blank in the vicinity of engagement with said dies wherein said axis of said helicoidal path is substantially coaxial with the axes of said helical path and said tubular blank.

11 A method of forming an integrally finned heat exchanger tube comprising the steps of: providing an elongated tubular blank of substantially circular cross section; supporting the internal surface of said tubular blank, and passing along a helical path relatTve to said tubular blank a series of fin extruding dies in engagement with the exterior surface of said tubular blank wherein the center of the areas of engagement lie substantially along a helicoidal path of changing diameter and pitch to accommodate changes in the length of said tubular blank in the vicinity of engagement with said dies wherein the axis of said helicoidal path is substantially coaxial with the axes of said helical path and said tubular blank.

12. A method of forming an integrally finned heat exchanger tube comprising the steps of: providing an elongated tubular blank of substantially circular cross section; supporting said tubular blank; and passing along a helical path relative to said tubular blank a series of fin extruding dies in engagement with the surface of said tubular blank wherein the centers of the areas of engagement lie substantially along a' helicoidal path of changing diameter and pitch commensurate with changes in the length of said tubular blank in the vicinity of engagement with said dies wherein the axis of said helicoidal path is substantially coaxial with the axes of said helical path and said tubular blank.

13. The apparatus as defined by claim 3 wherein said pitch increases at a generally decreasing rate from said leading end toward said trailing end.

14. The method as defined by claim 10 wherein said increasing pitch increases at a decreasing rate. 

1. Apparatus for forming a helical finned tube comprising; means for supporting an elongated tubular blank; means for initially forming a shallow helical groove in the surface of the tube blank, and die means including work engaging portions for simultaneously changing the depth and pitch of said groove to accommodate changes of length of the tubular blank in the vicinity of engagement with said die means.
 2. The apparatus as defined by claim 1 wherein said change of pitch of said helicoidal path is achieved at least once in each of a plurality of successive convolutions.
 3. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the pitch between the dies on separate roll assemblies and proceeding in a single groove in the outer surface of said tubular blank generally increases from the leading end toward the training end of the procession of dies in said single groove to accommodate elongation of said tubular blank in the vicinity of engagement with said roll assemblies.
 4. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the pitch between the dies on each roll assembly generally increases from the leading end toward the trailing end of the roll assembly to accommodate elongation of said tubular blank in the vicinity of engagement with said roll assemblies.
 5. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the leading die of one roll assembly is displaced forwardly of the leading die of another roll assembly and the average pitch between the dies of said one roll assembly is less than the average pitch between the dies of said other roll assembly to thereby accommodate elongation of said tubular blaNk in the vicinity of engagement with said roll assemblies.
 6. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the average pitch between the dies in the forward half of each of said roll assemblies is less than the average pitch between the dies in the rear half of each of said roll assemblies to accommodate elongation of the tubular blank in the vicinity of engagement with said roll assemblies.
 7. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the pitch between the dies on separate roll assemblies and proceeding in a single groove in the outer surface of said tubular blank differs to accommodate changes in length of the tubular blank in the vicinity of engagement with said roll assemblies.
 8. In apparatus for forming an integral helical fin on an elongated tubular blank including, a mandrel means for internally supporting said tubular blank, means for rolling a helical groove into the outer surface of said tubular blank having at least three roll assemblies circumferentially arranged about said tubular blank, each roll assembly having a plurality of axially spaced roll dies adapted to rollingly engage the exterior surface of said tubular blank, the improvement wherein the pitch between the dies of one roll assembly differs from the pitch between the dies of another adjacent roll assembly to accommodate changes in length of the tubular blank in the vicinity of engagement with said roll assemblies.
 9. A method of forming an integrally finned heat exchanger tube comprising the steps of: providing an elongated tubular blank of substantially circular cross-section; supporting the internal surface of said tubular blank; and passing along a helical path relative to said tubular blank a series of fin extruding dies in engagement with the exterior surface of said tubular blank wherein the center of the areas of engagement lie substantially along a helicoidal path of decreasing diameter and increasing pitch to accommodate for elongation of said tubular blank in the vicinity of engagement with said dies wherein said axis of said helicoidal path is substantially coaxial with the axes of said helical path and said tubular blank.
 10. A method of forming an integrally finned heat exchanger tube comprising the steps of: providing an elongated tubular blank of substantially circular cross-section; supporting the internal surface of said tubular blank; and passing along a helical path relative to said tubular blank a series of roll dies in rolling engagement with the exterior surface of said tubular blank wherein the center of the areas of engagement lie substantially along a helicoidal path of decreasing diameter and increasing pitch to accommodate for elongation of said tubular blank in the vicinity of engagement with said dies wherein said axis of said helicoidal path is substantially coaxial with the axes of said helical path and said tubular blank.
 11. A method of forming an integrally finned heat exchanger tube comprising the steps of: providing an elongated tubular blank of substantially circular cross-section; supporting the internal surface of said tubular blank, and passing along a helical path relative to said tubular blank a series of fin extruding dies in engagement with the exterior surface of said tubular blank wherein the center of the areas of engagement lie substantially along a helicoidal path of changing diameter and pitch to accommodate changes in the length of said tubular blank in the vicinity of engagement with said dies wherein the axis of said helicoidal path is substantially coaxial with the axes of said helical path and said tubular blank.
 12. A method of forming an integrally finned heat exchanger tube comprising the steps of: providing an elongated tubular blank of substantially circular cross-section; supporting said tubular blank; and passing along a helical path relative to said tubular blank a series of fin extruding dies in engagement with the surface of said tubular blank wherein the centers of the areas of engagement lie substantially along a helicoidal path of changing diameter and pitch commensurate with changes in the length of said tubular blank in the vicinity of engagement with said dies wherein the axis of said helicoidal path is substantially coaxial with the axes of said helical path and said tubular blank.
 13. The apparatus as defined by claim 3 wherein said pitch increases at a generally decreasing rate from said leading end toward said trailing end.
 14. The method as defined by claim 10 wherein said increasing pitch increases at a decreasing rate. 